Heat pipe with variable evaporator

ABSTRACT

A HEAT PIPE IS PROVIDED WITH MEANS IN THE EVAPORATOR SECTION FOR REDUCING THE EFFECTIVE CAPILLARY PORE DIAMETER OF THAT PORTION OF THE WICK. THE EFFECTIVE SIZE OF THE PORES IN THE EVAPORATOR WICK ARE VARIED IN RESPONSE TO AN EXTERNAL SIGNAL AND THE PRESSURE DIFFERENTIAL WHICH DRIVES THE CIRCULATING FLUID IS THEREBY CONTROLLED.

sept. 2o, 1971 J B- COLEMAN 3,605,878

HEAT PIPE WITH VARIABLE EVAPORATOR Filed 061". 8, 1969 EVAPORATORCONTROL M E ANS EVAPOR ATOR CONTROL MEANS l 2 2 INVENTOI? 2 JOHN B.COLEMAN AGENT United States Patent O 3,605,878 HEAT PIPE WITH VARIABLEEVAPORATOR John B. Coleman, Buffalo, N.Y., assignor to SandersAssociates, Inc., Nashua, N.H. Filed Oct. 8, 1969, Ser. No. 864,619 Int.Cl. F28d 15/00 U.S. Cl. 165-32 10 Claims ABSTRACT OF THE DISCLOSURE Aheat pipe is provided with means in the evaporator section for reducingthe effective capillary pore diameter of that portion of the wick. Theeffective size of the pores in the evaporator wick are varied inresponse `to an external signal and the pressure differential whichdrives the circulating fluid is thereby controlled.

BACKGROUND OF THE INVENTION Field of the invention The present inventionrelates generally to the field of heat transfer systems and inparticular to controlled thermal conductance pipes.

DESCRIPTION OF THE PRIOR ART Prior to the present invention if it weredesirable to vary the heat flux at a given temperature or thetemperature at a given heat flux in a heat pipe it was necessary tointroduce a volume of a non-condensable gas into the heat pipe tube toreduce the condenser area. This gas operated to control the temperatureof the vapors in the heat pipe but was not effective to control theterminal temperature nor is this approach amenable to proportional andindependent control in response to an externally applied signal. Furtherdisadvantages of this means of heat flux control revolve about thecomplexity of the plumbing apparatus required, the expense involved inits use, the size of the gas reservoir, and the insufficient accuracywith which the control could be effected.

OBJECTS AND SUMMARY OF THE INVENTION It is thus a primary object of thepresent invention to provide a new and novel controllable heat pipe.

It is another object of the present invention to provide apparatus ofthe above-described character having a proportionally controlledevaporation rate and/or terminal temperatures.

It is an additional object of the present invention to provide apparatusof the above-described character wherein the effective pore size of theevaporator wick is varied in response to an externally applied signal.

These and other objectives of the present invention are achieved byproviding a heat pipe having a generally cylindrical evaporator sectionwith a wick disposed about its inside surface. A cylindrical spiralspring is disposed inside the Wick such that in its uncompressed statethe spring has an outside diameter less than the inside diameter of theevaporator Wick. A spring compression means is provided which mayoperate in response to an external control signal to force the springradially against the Wick surface thereby reducing the effectivecapillary diameter. An increase in the vapor velocity in the evaporatorand an increase in the evaporation rate is thus provided.

The foregoing as well as other objects, features and advantages of thepresent invention will become more apparent from the detaileddescription taken in conjunction with the appended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. l is a schematic cross sectionalview of a con- 3,605,878 Patented Sept. 20, 1971 Ice DESCRIPTION OFPREFERRED EMBODIMENT Turning now to FIG. 1 there is illustrated inschematic longitudinal cross section a heat pipe evaporator sectionconstructed in accordance with the principles of the present invention.The remainder of the heat pipe is not shown in the interests of clarity.A generally cylindrical heat pipe tube 10 has a conventional fixed wiremesh ltype wick 12 disposed around its inner surface. Also conventionalin form is a source of heat flux 14 disposed in a heat transferrelationship to the tube 10. Heat from the source 14 operates toevaporate a liquid phase working fluid from the wick 12 and the vaporsmove under a slight pressure differential toward the condenser end (notshown) of the heat pipe. One of the significant factors inuencing theperformance of a heat pipe is the rate at which evaporation of liquidfrom the wick 12 occurs and a means for externally controlling thisfactor permits control of the heat flux and/or temperature at any pointin the heat transfer system. To this end the present invention includesa cylindrical spiral spring 16 having a spacing between the adjacentturns in the uncompressed state which is equal to or greater than theWire mesh 12 size. When the spring 16 is compressed, it reduces theeffective capillary pore diameter of the wick 12, thus decreasing theradius of curvature of the liquid meniscus. The pressure differentialacross the meniscus is thereby increased resulting in an increase invapor velocity and evaporation rate. The spring 16 is compressed inresponse to an externally applied control signal. In the embodimentshown in FIG. l the compression of spring 16 is effected by a magneticslug 18 disposed within the heat pipe tube 10 and which moves axiallyalong the tube under a magnetic field set up by an external solenoidcoil 20 driven by an evaporator control means 22. The force exerted onthe slug 18 and thus the amount of compression of the spring 16 isapproximately proportional to the square of the electric current fromthe control means 22 passing through the coil 20. In this manner theeffective size of the pores in the evaporator wick 12 surface is variedby an independently acting means, thus controlling the pressuredifferential Iwhich drives the circulating fluid. It will be apparentthat the evaporator control means 22 may be coupled to and controlled bythe output of a temperature sensor 24 affixed to the heat flux source 14thereby providing a simple, independent, proportional control of heatflux and/or temperature'. Further the evaporator control means 22 may bemade responsive to other signals derived from any other variable in theheat transfer system and may be coupled through an appropriateservomechanism to provide a closed control loop.

FIG. 2 schematically illustrates an alternative embodiment of thepresent invention wherein elements common to those shown in FIG. 1 areidentified with like reference numerals. In this embodiment of theinvention the heat pipe tube 10, evaporator wick 12, heat fiux source14, and cylindrical spiral spring 16 are identical in structure andfunction to the like elements described hereinabove with reference toFIG. 1. In this case, however, spring 16 compression is achieved by apiston 26 coupled by an actuating rod 28 to a hydraulic, pneumatic orthermal expansion actuator 30. The axial distance through which theactutaor 30 drives the piston 26 is determined by the electrical outputsignal from an evaporator control means 22. As discussed hereinabove theevaporator control means 22 may be coupled to a temperature sensor (notshown) to thereby provide closed loop proportional control of the heatpipe evaporator.

It will be apparent that the applicant has provided a new and novel heatpipe having means for the proportional control of the evaporation rate,the heat flux, or any other measurable variable in the heat transfersystem, in response to an externally applied signal. The objectives setforth above are thereby eciently met and, since certain changes may bemade in the above-described construction without departing from thescope of the invention, it is intended that all matter contained in theforegoing description or shown in the appended drawings shall beinterpreted as illustrated and not in a limiting sense.

Having described what is new and novel and desired to secure by LettersPatent, what is claimed is:

1. An improved heat transfer apparatus of the type characterized asincluding a closed container having an evaporator section coupled to aheat source, a condenser section coupled to a heat sink, and a capillarystructure saturated lwith a volatile fluid and disposed on the interiorsurfaces of said container and wherein said improvement comprises aspiral spring disposed within and coaxially aligned 4with said capillarystructure,

means for providing axial compression of said spring to thereby radiallycompress said capillary structure, and

means adjacent said spring for restricting axial movement thereof andagainst which said spring may be compressed.

2. Apparatus as recited in claim 1 wherein said capillary structure is amesh wick.

3. Apparatus as recited in claim 2 wherein the spacing between turns ofsaid spring in its uncompressed state is no greater than the size ofsaid mesh.

4. Apparatus as recited in claim 2 wherein said spring compressing meanscomprises a magnetic slug disposed within said evaporator section ofsaid container in axial alignment with said spring,

a solenoid coil disposed externally about a portion of said evaporatorsection of said container, and

a variable source of electric current coupled to said coil wherebyspring compression proportional to the square of said electric currentis provided.

5. Apparatus as recited in claim 4 further including a thermal sensorcoupled to said heat transfer apparatus and having the output thereofcoupled to and operative to control the output of said current sourcewhereby automatic proportional control of the thermal conductance ofsaid heat transfer apparatus is provided.

6. Apparatus as recited in claim 2 wherein said spring compressing meanscomprises a piston disposed within said evaporator section of saidcontainer in axial alignment with said spring piston actuating meansdisposed external to said container and coupled through said containerto said piston, and

means for controlling the output pressure of said actuating means.

7. Apparatus as recited in claim 6 further including a sensor coupled tosaid heat transfer apparatus and having the output thereof coupled toand operative to control the output of said pressure controlling meanswhereby automatic proportional control of the thermal conductance ofsaid heat transfer apparatus is provided.

8. Apparatus as recited in claim 6 wherein said actuating means is ahydraulic actuator.

'9. Apparatus as recited in claim 6 wherein said actuating means is apneumatic actuator.

10. Apparatus as recited in claim 6 wherein said actuating means is athermal expansion actuator.

References Cited UNITED STATES PATENTS 3,414,050 12/1968 Anand 165-105X3,502,138 3/1970 Shlosinger 165-105X 3,516,487 6/1970 Keiser 165--1053,517,730 6/1970 Wyatt 165-105X 3,519,067 7/1970 Schmidt 165-105X 40ALBERT w. DAVIS, JR., Primary Examiner U.S. Cl. X.R.

